A display screen is a memory display screen if its electro-optical characteristic (luminance-voltage curve) exhibits hysteresis. For a given voltage located inside the hysteresis loop, the device may thus have two stable states: switched off (unlit) or switched on (lit).
The advantages of a memory effect display are considerable: so as to display a fixed image, it merely suffices to simultaneously and continuously apply to any screen a voltage, known as a holding voltage. This voltage can be a sinusoidal signal or a signal in the form of strobes, but in particular the form and frequency of this holding signal can be selected independently of the complexity of the screen. Thus, in principle, there is no limit to the complexity of a memory display screen. Accordingly, bistable plasma screens and alternative excitation screens with 1200.times.1200 picture elements (pixels) are commercialized.
Furthermore, the technology of display by electroluminescence in thin films and with capactive coupling (ACTFEL for short) has now been fully evolved within industrial applications. These devices can be provided with an inherent memory effect, but at the cost of significantly deteriorating electro-optical performances. A more promising method consists of connecting a photoconductive structure (PC) in series with an electroluminescent structure (EL) and to optically couple these two structures.
Thus, it is possible to produce an extrinsic type memory effect known as a PC-E1 memory effect whose principle is the following: when the device is in the switched off state, the photoconductive material is slightly conductive and retains, a significant part of the voltage V applied to the unit. If V is increased to a value Von so that the voltage present at the terminals of the electroluminescent structure exceeds an electroluminescence threshold, the device PC-E1 tilts into the switched on state. The photoconductive material is then lit up by the electroluminescent structure and passes to the conductive state. The voltage at its terminals drops and as a result the voltage available for the electroluminescent structure increases. In order to switch off a PC-E1 device, it merely suffices to reduce the total voltage V to a value Voff less than Von: thus, a luminosity/voltage comprising an hysteresis is obtained.
A PC-E1 structure was recently described in the document FR-A-2 574 972 and in the article of the inventor and entitled "Monolithic thin-film photoconductive-ACEL structure with extrinsic memory by optical coupling" published in IEEE Transactions on Electron Devices, vol. ED-33, No. 8, August 1986, pages 1149-1153.
This structure is diagrammatically shown as a sectional view on FIG. 1. It includes a glass substrate 10 on which deposited are a transparent electrode 12, a first dielectric film 14, an E1 electroluminescent film 16, a second dielectric film 18, a PC photoconductive film 20 and finally a reflecting electrode 22. In this embodiment, the PC and E1films are thin films whose thickness is about one micrometer. The electrodes 12 and 22 are connected to an alternating or a.c. voltage 24.
For a matrix display, electrodes 12 are used, as shown on the top part of FIG. 2, said electrodes constituted by strips or groups of conductive strips parallel to each other and electrodes 22, also constituted by strips or groups of conductive strips parallel to each other, the electrodes 12 being perpendicular to the electrodes 22. The electrodes 12 and 22 indifferently play the role of line electrodes or column electrodes and are connected to control circuits 23 and 25.
Such a structure is embodied relatively simply as it does not involve any additional engraving stages. In addition, the current/voltage behaviour of the thin film photoconductor in darkness is highly non-linear and reproduceable. The favorable consequences are that the electric lighting up of the device is still relatively simple, that the hysteresis only slightly depends on the excitation frequency and that the reproductibility of the hystresis margin from one production to another is guaranteed.
In a recent publication of P. Thioulouse from the Proceedings 4th International Workshop on Electroluminescence E1-88, Tottori (JP), 11-14 Oct. 1988 and entitled "Thin film photoconductor electroluminescent memory, display devices", an improved structure of the PC-E1device described above is proposed, said device being diagrammatically represented as a sectional view on FIG. 3. In this structure, the PC film 20 draws closer to the emitting film 16 by almost fully transferring the non-conducting film 18 situated between them above the photoconductive film 20. A fine non-conducting film 27 is then left between the films 16 and 22 so as to protect the emitting film 16.
The main advantages of this new structure are the following: better optical coupling between the films E1 16 and PC 20, a virtually annulled optical guidance in the emitting film and clearly improved operating reliability with respect to the device of FIG. 1 (cicatrisation of electrical disruptive breakdowns).
The present invention is mainly applicable to this new structure.
Furthermore, it is sought to generally maximize the resistively of the PC film so as to avoid any lateral parasitic conduction (known in planar conduction terminology), the inventor has proposed and established this increase of resistivity by selecting an alloy a-Si.sub.1 -.sub.x C.sub.x : H as a PC photoconductive material instead of the a-Si: H normally used (as regards this subject, refer to the document FR-A-2 105 777 and the aforementioned article Proceedings).
Thus, it is possible to reduce the conductivity of the intrinsic PC photoconductive material from 10.sup.-6 to 10.sup.-13 (.OMEGA..cm).sup.-1. The intrinsic photoconductive film marked i then becomes as resistive as the other films of the PC-E1 structure.
As described in the above-mentioned articles of the inventor and illustrated diagrammatically on FIG. 3, the PC photoconductive film is generally composed of a stacking of n.sup.+ -i-n+ films. The two films n.sup.+, also with an amorphous hydrogenated silicon base, are obtained by doping with phosphorus (phosphine being added during the depositing) and aim to make it possible to inject an electronic quasi-ohmic substance into the intrinsic film.
These films n.sup.+ are significantly more conductive than the intrinsic films i and the incorporation of carbon into the material of the films nm.sup.- (a-Si.sub.1-x C.sub.x : H) also makes it possible to considerably reduce their conductivty, usually by 10.sup.-2 -10.sup.-3 to 10.sup.-5 -10.sup.-6 .OMEGA..sup.-l 1.cm.sup.-1. Unfortunately, the films n.sup.+, even those containing carbon, are still sufficiently conductive so as to provoke the parasitic phenomenon to be described subsequently.